With a rocket getting to say 100,000' in the shortest period of time is an important factor; one has to get above Earth's dense atmosphere before reaching speeds needed for orbital or escape velocity. When one has rockets which has stages, Kerosene & LOX is a preferred rocket fuel for first stage with Liquid Hydrogen & LOX [LH&LOX] a better rocket fuel for the second and/or third stages.
This is because Kerosene is a denser source of energy- it require less volume- which means a smaller engine, fuel tanks and plumbing give more power. Whereas in terms of it's mass/weight LH&LOX gives more power per weight than Kerosene LOX and it has a significantly higher ISP].

In order to have a space frontier which is open, markets will be needed in space. There are lots things which could bought or sold in space, but the most important commodity that is required to open the space frontier is the ability to buy and sell rocket fuel.
And for rocket fuel in space to sell, it needs to sell at cheap price. For example a cheap price for rocket fuel in LEO is at or below $2000 per lb.

When there are markets in space, and people are going to such places as the Moon and Mars, Mars could be the cheapest places to buy rocket fuel beyond Earth and that rocket fuel will probably be Methane and LOX, but Liquid Hydrogen and LOX [LH&LOX] would probably also be sold.

Methane [CH4] on Mars is easy to make IF you have Hydrogen- because one can use the CO2 of the atmosphere to make methane from Hydrogen.
Methane is similar to Kerosene- in that it's a denser fuel than Hydrogen. Another aspect about Mars is there is *relatively* a large amount of water which could mined on Mars. For purposes of fuel for rockets, a billion tonnes of water is a lot of water- but in comparison a small lake on Earth could have a billion tonnes of water- or a heavy rainfall could deliver a billion tonnes of water to a large area [i.e 500 km square is 250,000 sq km and with average 1 centimeter of rain that would be 2.5 billion tonnes of water- a Hurricane could delivery tens to hundred billion tonnes].

So, Earth has a lot of water, Mars in comparison is a very dry and airless world. On Earth you generally pump water from well or channel water through pipes from water reservoirs at higher elevation. But on Mars you will have to mine it, one might mine nearly pure water ice at the poles or extract frozen water mixed with relogith, perhaps drilling wells in which water maybe fairly close to surface or miles beneath it. Or it's possible that water may be found which is geothermally heated so it can be near the surface and be liquid.

Though hundreds of trillion of tonnes fresh water can found in lakes on Earth and far more can found as frozen ice in glaciers or polar region- these are all dwarfed by the amount saltwater in the oceans.

But in terms of our entire solar system all the water on earth including it's vast oceans is probably less than a millionth of all solar system's water. There is probably more water on Jupiter's moon Europa than on Earth, and Europa is only one of major Jupiter moons that has large quantities of water. But even if you exclude all the outer planets and their moons, the Kuiper Belt, and Oort Cloud, and are just talking about the inner part of the system, there is probably more water there, than on Earth.

Example:
"The total volume of water on Earth is about 1.4 billion cubic kilometers, around 41 million of which is fresh water. If Ceres' mantle accounts for 25 percent of the asteroid's mass, that would translate to an upper limit of 200 million cubic kilometers of water, Parker said."
http://www.space.com/scienceastronomy/050907_ceres_planet.html

So, a billion tonnes is a lot of water if you want to use it for rocket fuel. Mars could have a trillion tonnes which are minable- and the Moon may have a billion tonnes or so which are minable- some scientists think they could be 10 billion tonnes of water at both of the Moon's poles.

Let's try a hypothetical, assume there are mining companies mining lunar water, and you mining companies mining Martian water, and you have a buyer of water who wants to buy one hundred thousand tonnes of water. In other words, the buyer wants one hundred thousand tonnes of water delivered on anyplace on Mars or Lunar surface, the buyer will provide the container, and miners deliver the water to the container. How much per cubic meter [tonne] of water will the miners give as a price? Perhaps, the mostly likely result will be that Maritan water miner can give a lower price.

So, this buyer of water could buying the water in order to make rocket fuel so he/she would also need to buy electrical power. It takes about 4 kwh of energy per kg of water, but with conversion losses and energy needed to pumping and liquifying, ect, we will take a round number of 8 kwh of energy per kg, or 8000 kwh per tonne. So if you wanted a tonne of rocket fuel made per hour you would need constant source of 8 megawatts of power- and at this rate it would take 100,000 hours [11.4 years].

So, a question could be how long does it take for water miner to deliver the 100,000 tons of water and how quickly can a certain amount power can be provided and at what price?

Now, the buyer does not want to hold 100,000 tonnes of water, and then spend 11.4 years to process it, nor hold 100,000 tons of rocket fuel then sell it.

Instead it would cost less if there was on average only about 100 tons of water, and 100 tons of rocket fuel and rocket fuel was being made about as fast as it was sold, and instead taking 11.4 years, it would be more profitable if instead it takes 5 years to get the 100,000 tons of water, process it, and sell it, and buy more water and repeat, except perhaps this time you want to buy a million tonnes of water and process and sell it all within 2 years.

So, a buyer of water, could be looking at the next ten years, though buying 100,000 tonnes, within the 10 years may want to be able to buy +1 million tonnes of water, and though may start with say 4 mwh, within 10 year may want to able to buy +100 mwh. Of course needs a potential market that within 10 years be able to sell a million tonnes of rocket fuel.

Ok, so say Martian miner or Lunar miner has capability to mine 100,000 tonnes within 5 years, and could sell water at $500 per lb or 1.1 million per tonne [so gross value would be 110 billion] so in 5 year spend 50-60 billion in order to make 110 billion. Say of this cost, 20 billion dollars is spent in transportation costs- lift from Earth to the Moon or Mars and any return to earth in regards to say personnel. And 1/2 of that transportation cost is leaving earth, and say this cost is $1000 per lb, so that gives a max of 10 million lbs lifted from Earth [now, of that 10 million lbs, if one could buy any of the stuff needed from a supplier on the Moon or Mars, one of course would be willing to pay at least $1000 per lb for it- though anything cheaper is obviously preferred]. Now, obviously the water miner will not need to bring rocket fuel from Earth to return to Earth, but also the miner will not need to bring fuel from earth to land on the Moon or Mars because one could/would have gas station in orbit in water or rocket is provided from Moon or Mars.

Now, some people in the past have had the crazy idea of making electrical power on the Moon and beaming that power back to earth and selling it for something like 10 cent per kwh to Earthlings. Our water buyer would happy if he could only spend hundred times more, say $10 per kwh, for the quantity power up to 100 mwh if within 10 years, and within a couple decades 10 times that amount. Or said differently, a need for about 10 nuclear reactor build and operating within a decade. Or assuming one could get 280 watts per sq meter in solar power, 10,000 sq meter might do for first few months, but within a year, so somewhere around sq kilometer of solar panels might be needed [280 mwh].

If you could buy power for $10 kwh, then you could probably mine and make Aluminum for less than $1000 per lb- energy cost per lb to convert Aluminum oxides to Aluminum would be about $68 per lb.

Of course if you making Aluminum you also going to have Oxygen as by product. Now, solar power will receive on the Moon at least twice as much power on Moon as on Mars [Mars because of it's distance receives from the sun 40% of solar energy]. And in the polar region there are locations which are nearly always in sunlight [therefore one could get 4 times the amount of solar energy per sq meter as compared to Mars]. Therefore, to get longer sunlight, and sunlight in more location, someone who makes Aluminum on the Moon could provide Aluminum that could used to make towers for solar panel. And things like frames and wiring can made from Aluminum.

If electricity can be bought for $10 kwh and water for $500 lb, that means it takes $80 energy to make a lb of rocket fuel [9 lbs of water makes 1 lb of Hydrogen and 8 lbs of Oxygen], so it's possible that rocket fuel could be sold for $1000 per lb on the lunar surface. And using that rocket fuel one could lift rocket fuel or water to lunar orbit, and sell rocket fuel in lunar orbit for around $2000 per lb. Whereas getting rocket from *Earth* to lunar surface might cost +$10,000 per lb and getting rocket fuel to Lunar orbit might cost around +$5000 per lb. So making rocket fuel on the Moon easily halves the cost of landing on the Moon and if want to leave the moon or export anything from the Moon it's AT Least 1/10th the cost.

Wrapping this up, in an apple to apple comparison, in this general situation one could have water sold in Earth orbit for $500 lb. And water on Moon sold for $500/lb and water sold on Mars for $100/lb. Electrical power in Earth orbit for $20 per kwh, $10 per kwh on Moon, and $40 per kwh on Mars. Rocket fuel in Earth orbit $1500 per lb. Lunar surface: $1000 per lb, Lunar orbit $2000 per lb. And on Mars rocket fuel could cost $500 per lb and in Mar's orbit $2000 per lb.
And as market force drive down costs, you will have water on the Moon or Mars as cheap as on than Earth and energy much being much cheaper than on Earth- and exporting electrical power to Earth.